Bottles for culturing of blood for the presence of microorganism and related instruments for analyzing such bottles in a noninvasive manner are known in the art and described in the patent literature. See U.S. Pat. Nos. 5,858,769; 5,795,773; 4,945,060; 5,094,955; 5,164,796; 5,217,876; and 5,856,175. The bottles and instruments of the above-listed patents have been commercialized with success by the present assignee under the trademark BacT/ALERT.
The bottles described in these blood culture instruments utilize colorimetric sensors placed in the bottom of the bottle and in contact with the sample media to determine the presence/absence of bacterial growth. Once a clinical/industry sample is added to the liquid growth media present in the bottle and incubation occurs, the concentration of carbon dioxide increases as the number of microorganisms increase; carbon dioxide is a respiration by-product of bacterial growth. Alternatively, changes to the media pH that are related to the growth of microorganisms can also be monitored by the sensor. The basic operation of the BacT/ALERT sensor and monitoring electronics is described in U.S. Pat. No. 4,945,060 and also in an article by Thorpe et. al. in “BacT/Alert: an Automated Colorimetric Microbial Detection System” which was published in the Journal of Clinical Microbiology, July 1990, pp. 1608-12. The '060 patent and the Thorpe et al. article are incorporated by reference here.
The basic colorimetric sensing system described in the '060 patent is shown in FIG. 1 of the appended figures. A red Light Emitting Diode (LED) (4) shines onto the bottom of the BacT bottle (1). A colorimetric sensor (2) is deposited onto the bottom of the bottle (1). The LED light impinges on the sensor at a 45 degree angle relative to the bottom surface of the bottle (1). The majority of the light penetrates the structure of the bottle and impinges on the colorimetric sensor (2). Part of the light will reflect off the plastic bottle material and sensor (2) at 45 degrees to the bottom surface of the bottle, but in an opposite direction to the impinging light (e.g. the angle of reflection is equivalent to the angle of incidence). Much of the remaining light is scattered from the surface and interior of the sensor. The sensor (2) changes its color as the percentage of CO2 in the bottle varies from 0% to 100%; the color varies from blue to yellow, respectively. A silicon photodetector (5) “stares” (i.e., continuously monitors the scattered intensity signal) at the region in the sensor (2) where the light from the LED interacts with the sensor. The intensity of the scattered light that is detected by the photodetector is proportional to the CO2 level within the bottle (1). FIG. 1 also shows the associated electronics including a current source (6), current-to-voltage converter (7) and low pass filter (8).
FIG. 2 is a plot of the signal received by the photodetector (5) of FIG. 1. The data was collected using a fiber optic probe in place of the photodetector (5) in FIG. 1. The fiber optic probe is routed to a visible light spectrometer, which shows the scattered light as a function of intensity (Reflectance Units) and wavelength. The shape of each curve is the convolution of the LED intensity distribution with the reflectivity of the colorimetric sensor (2) at a specified CO2 level.
When the silicon photodetector (5) of FIG. 1 is substituted for the fiber optic probe, a photocurrent is generated by the photodetector that is proportional to the integrated wavelength signal shown in FIG. 2. In other words, the silicon photodetector integrates the spectral response into a photocurrent. In turn, this photocurrent is converted into a voltage signal using a transimpedance amplifier.
While the BacT/ALERT sensing system of FIG. 1 is robust and has been used in blood culture systems successfully for many years, it does have a few areas for improvement. First, if the blood culture bottle (1) moves in the cell (e.g. displacement in the z-axis so that it shifts away from the position of the photodetector), the system (as it is currently implemented) detects this movement as a reduction in intensity. However, this reduction in intensity is interpreted by the instrument as reduction in CO2 level in the bottle, which may not in fact be occurring. Since this effect is counter to the effect of a bottle's reflectivity increasing as carbon dioxide content increases (signifying bacterial growth), it is possible that the system would treat a translating bottle as having no growth (i.e., a false negative condition).
Likewise, as the instrument ages in the clinical laboratory, the optical system may collect dust or optical materials experience reduced transmissivity as a function of time. For example, as plastics age, their transmissivity can be reduced by the effects of light, particulate buildup (dust) or repeated use of cleaning agents. These effects would not affect readings but would manifest as a drift in the response of the system. Periodic calibration checks could compensate for this drift. Thus, there is a long-felt but unmet need to have a real-time monitor of the transmission in the optical system and the capability to adjust or compensate for some of these sources of error, particularly the situation where the bottle is not fully installed in the receptacle and is not at the nominal or home position (has some Z-axis displacement away from the optical detector arrangement).
Other prior art of interest includes the following U.S. Pat. Nos. 7,193,717; 5,482,842; 5,480,804; 5,064,282; 5,013,155; 6,096,2726; 6,665,061; 4,248,536 and published PCT application WO 94/26874 published Nov. 24, 1994.